Biomass refining (or biorefining) is becoming more prevalent in industry. Cellulose fibers and sugars, hemicellulose sugars, lignin, syngas, and derivatives of these intermediates are being used by many companies for chemical and fuel production. Indeed, we now are observing the commercialization of integrated biorefineries that are capable of processing incoming biomass much the same as petroleum refineries now process crude oil. Underutilized lignocellulosic biomass feedstocks have the potential to be much cheaper than petroleum, on a carbon basis, as well as much better from an environmental life-cycle standpoint.
Lignocellulosic biomass is the most abundant renewable material on the planet and has long been recognized as a potential feedstock for producing chemicals, fuels, and materials. Lignocellulosic biomass normally comprises primarily cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are natural polymers of sugars, and lignin is an aromatic/aliphatic hydrocarbon polymer reinforcing the entire biomass network. Some forms of biomass (e.g., recycled materials) do not contain hemicellulose.
Lignocellulosic biomass presents the vast majority of the sustainable feedstock for sugar-based fermentation. Lignocellulosics include woody and herbaceous material, and residues left behind after forestry and agricultural harvesting and processing. Biomass utilization for sugar recovery is hindered by its recalcitrant nature, resisting deconstruction by most chemicals and microorganisms. Even if the sugars are extracted, they come in a mixture of pentoses and hexoses. Most microorganisms can only utilize hexose sugars, or perform poorly on pentose conversion. Furthermore, the degradation products from harsh pretreatment, or the pretreatment chemicals themselves, are normally toxic to the microorganisms. These impurities can limit the conversion yield, selectivity, and productivity.
Often the rate-limiting step of fermentation is that the product itself inhibits the microorganism (e.g., Clostridium) productivity. Furthermore, the fermentation is normally done in a batch process, charging new microorganisms to each batch of sugars. Using membrane technology developed by Toray Industries of Japan, the fermented solvents can be passed through a membrane, where the Clostridium is recycled back to the fermentor and the products are sent to downstream purification. Continuous membrane-assisted fermentation enables many fold volumetric productivity enhancement, reducing the cost of fermentors.
Liquid-liquid extraction has been used to recover solutes that have strong solubility into the extractant. Concentrated solute is recovered from the extractant. The liquid-liquid extraction can significantly reduce the energy consumption, compared to traditional distillation. Energy integration and heat recovery can further reduce the energy consumption—making it even less than traditional ethanol production.
Clostridium acetobutylicum has been used for industrial production of butanol and acetone since World War I, where acetone was needed for cordite production. By 1950, about two-thirds of the U.S. production of butanol was made by fermentation from starch or molasses (Dodds, 2017). Upon emergence of efficient petrochemical butanol production processes, only a few butanol plants remained in production in the former Soviet Union, Egypt, South Africa, and China until late 20th century. Biomass feedstock was contemplated by the Soviet researchers in the 1960s with mild sulfuric acid to hydrolyze pentoses from agricultural waste (Zverlov et al., 2006). The “continual fermentation” was also established using parallel batteries of 4-8 fermentors improving solvent productivity by 31% over batch fermentation.
Renewed interest in alcohols in the U.S. was driven by elimination of methyl tertiary-butyl ether (MTBE) as an oxygenate in gasoline in 2006 in the U.S., which rapidly was replaced by corn-derived ethanol. Recently two corn ethanol plants in Minnesota have converted to butanol production in the U.S. to produce a higher-value product. The alternative oxygenates including propanol, butanol, pentanol, hexanol and their isomers contain more energy and less oxygen, which allows higher blending in the gasoline. Butanol has a lower blend octane rating than propanol and ethanol. A customized mixture of the alcohols can used to obtain desirable blending characteristics.
The fermentative production of alcohols has only seen incremental productivity improvements. The shortcomings of current biofuel production are well-documented including long biological processing times and energy intensity owing to dilute aqueous solutions. Typical fermentation duration from 24 hours to 72 hours and low titer leads to large industrial fermentation vessels. The long residence times make fermentation susceptible to infection and require sterilization between batches. This further extends the cycle time and reduces capacity utilization. Typical productivity in industrial batch fermentation is between 0.2 and 0.5 g/L/h in fermentation vessels exceeding 1 million liters.
The industrial recovery of ethanol and butanol is being performed almost exclusively using steam stripping. Ethanol feed (to the purification section) contains between 6 vol % and 20 vol % of alcohol, while butanol feed (to the purification section) is only about 2 vol %. This large amount of water must be heated and recycled in the process. The water recycle from corn stillage is commonly accomplished using evaporation, where byproduct dry distillers grains and solubles (DDGS) are recovered. The butanol stillage is much more dilute and anaerobic digestion is typically used for water reuse. Anaerobic digester consumes organic matter and nutrients without providing significant value to the plant. The specific energy consumption of the first-generation product recovery is at least half of the energy contained in the alcohol product, and in the worst case exceeds it.
Lignocellulosic feedstocks, genetic engineering, new process solutions, and design innovations have been suggested to improve sustainability of fermentative processes. However, very little information has been published on the scale-up results and commercialization efforts. Nimcevic and Gapes (2000) published pilot-scale production experience in Austria, where they pointed out that the reliability was one of the reasons that industrial plants utilized batch operation. Butanol production is disadvantaged because of rigorous sanitary requirements, comparative thermal inefficiency, and large amount of water effluent after distillation of product.
Continuous fermentations with enhanced product concentration and productivity have been studied in literature such as continuous chemostat cultures (Gapes et al., 1996), immobilized packed bed reactors (Qureshi et al. 2000; Survase et al. 2012), immobilized fibrous bed reactors (Huang et al., 2002) and membrane-assisted high cell-density continuous cultures (Jang et al., 2013; Tashiro et al., 2005). Tashiro also stated the significance of cell-density control in high-cell-density bioreactors with cell recycling. During the cell recycling, the bioreactor faced a problem controlling the volume of broth in the reactor due to the heavy gas formation and high viscosity.
Fermentors with cell recycling require significant time to build a high cell density. Ferras et al. (1986) reported the requirement of more than 100 hr of cultivation to achieve a cell concentration greater than 20 g/L. To reduce this time, Tashiro et al. (2005) and Zheng et al. (2013) first concentrated the cells of the broth 10 times. A high cell density of approximately 20 g/L was obtained after only 12 h of cultivation by this operation, and the acetone-butanol-ethanol (ABE) productivity increased to greater than 10 g/L/h in short time.
Ni et al. (2013) studied corn stover hydrolysate and cane molasses for butanol fermentation by C. saccharobutylicum DSM 13864 in continuous fermentation. They reported that using cane molasses and corn stover hydrolysate as substrate, total solvents of 13.75 g/L and 11.43 g/L were obtained, respectively. The solvent productivities were 0.439 g/L/h and 0.429 g/L/h in a four-stage continuous fermentation continuously operated for 220 hr without compromise in solvent titer.
The use of hemicellulosic spent liquor from SO2-ethanol-water fractionation with supplemented glucose was demonstrated using immobilized packed bed column reactor by Survase et al. (2011). They reported maximum productivity of 4.86 g/L/h with 7.6 g/L total solvents and 0.27 g/g of solvent yield.
Besides an efficient fermentation, the product recovery cost and purification are important to make commercially viable biochemicals. The proposed methods for the recovery of butanol include adsorption (Xue et al., 2016), liquid-liquid extraction (Bankar et al. 2013), gas stripping (Cai et al. 2016), vacuum fermentation (Mariano et al. 2012), and pervaporation (Cai et al. 2017). Integration of solvent recovery can reduce the solvent toxicity significantly and improve the substrate consumption. To overcome the toxicity of the solvents, especially by n-butanol, investigations on the highly selective water-immiscible extractant to remove solvents was shown to increase the solvent titers and yields (Bankar et al., 2012; Bankar et al., 2013). The academic literature proposed several extractants for the butanol extraction including oleyl alcohol, decanol, benzyl benzoate, butyl phthalate (Qureshi and Maddox, 1995, Bankar et al. 2013), 1-dodecanol (Tanakaa et al., 2012), poly(propylene glycol) 1200 (Barton and Daugulis, 1992), castor oil, and oleic acid (Groot et al., 1990). Butyl butyrate, as taught by Melin et al. in WO2015193553, is an extractant with high distribution coefficient, especially for butanol, and low solubility with water.
The addition of acetate and butyrate into the culture media was found not only to enhance solvent production, but also to affect the ratio of acetone/butanol, which might result from the metabolic changes in solvent production (Lee et al., 2008). Gyamerah and Glover (1996) constructed a continuous pilot plant for fermentative production of ethanol, using liquid-liquid extraction to remove the product and recycle the raffinate. They used n-dodecanol as an extractant and immobilized yeast was used to overcome the problem of emulsification. The concentration of byproducts in the fermented broth because of recycle had no adverse effect on the rate of ethanol production. The raffinate recycle allowed higher feed glucose concentration (45.8% w/w) and reported 78% reduction in aqueous purge compared with using a feed containing 10% (w/w) glucose. The effluent recycle after removal of butanol by pervaporation resulted in 101.4% sugar utilization in addition to high productivity of 16.2 g/L/h at a dilution rate of 2.0 per h. A continuous immobilized cell (biofilm) plug-flow reactor with Clostridium beijerinckii BA101 was used (Lienhardt et al. 2002).
Improvements are needed in the art, in particular, to deal with low-quality hemicellulose sugars (e.g., C5 sugars) derived from biomass.